1. Technical Field
The present invention relates to an atomic oscillator, especially to a mounting technique of a gas cell included to an atomic oscillator.
2. Related Art
Atomic oscillators using alkali metals such as rubidium and cesium operate while maintaining a gas cell, in which atoms are air-tightly sealed, at a high temperature because the atoms need to be kept in a gas state when the oscillators use energy transition of the atoms. An operating principle of the atomic oscillators is broadly classified into a double resonance method utilizing light and micro waves as disclosed in JP-A-10-284772 as a first example and a method utilizing quantum interference effect produced by two kinds of laser light (hereinafter, referred to as coherent population trapping: CPT) as disclosed in U.S. Pat. No. 6,806,784 as a second example.
FIG. 6A shows a structure of a related art atomic oscillator adopting the CPT. An atomic oscillator 50 includes an optical system in which a semiconductor laser 52, a gas cell 54, and a light detector 56 are formed in a unified manner as disclosed in the second example. In the gas cell 54, alkali metal atoms (not shown) such as a rubidium atom and a cesium atom that are quantum absorbers are sealed. The semiconductor laser 52 produces two kinds of laser light (coupling light and probe light) having different wavelengths from each other and outputs the laser light to the gas cell 54. The atomic oscillator 50 detects how much laser light made incident to the gas cell 54 is absorbed by metal atom gas with its light detector 56 so as to detect atomic resonance, and allows a reference signal of a quartz crystal oscillator and the like to synchronize with the atomic resonance at a control system such as the frequency control circuit 58, obtaining an output. The light detector 56 is positioned at an opposite side of the side, to which the light is made incident, of the gas cell 54.
FIG. 6B shows energy levels of the quantum absorbers. The energy levels of the quantum absorbers include two ground levels (a ground level 1 and a ground level 2) and a three-level system (A type level system, for example) having an excitation level. When a difference between two frequencies (ω1 and ω2) of the resonance light that is simultaneously radiated precisely matches an energy difference between the ground level 1 and the ground level 2, the three-level system can be expressed by the coherent state between the ground level 1 and the ground level 2. That is, the excitation to the excitation level is stopped.
Namely, as shown in an optical absorption spectrum of FIG. 6C, the quantum absorbers in the gas cell 54 absorb the laser light radiated from the semiconductor laser and an optical absorption property (transmission) varies depending on frequency difference between the two kinds of light. In a case where frequencies of the coupling light and the probe light have specific values, neither two kinds of the light are absorbed but transmit (electromagnetically induced transparency (EIT) phenomenon). The CPT uses this EIT phenomenon so as to detect and use a state, in which the light absorption is stopped in the gas cell when one of or both of the two wavelengths are varied, as an EIT signal (refer to FIG. 6C) having a shape like δ function. In the second example, collimated semiconductor laser (dual wavelength having an energy difference of an hyperfine structure of the alkali metal atoms in a ground state) is made incident from a light incident window on a gas cell in which alkali metal atoms are sealed.
In the optical system of the related art atomic oscillator 50 shown in FIG. 6A, a beam radius of the laser light is substantially smaller than a cross-section area of the gas cell 54. Therefore, the laser light interacts with part of the atoms in the gas cell 54 without changing its laser radius, travels straight through a light emitting window, and reaches a part of the light detector 56 that is opposed to the light emitting window.
In such structure, the laser light performs the light-atom interaction only with the part of the metal atoms on an optical path in the gas cell 54, whereby almost all of the metal atoms uselessly exist. Further, if the laser radius is small as this, the metal atoms passing across the laser light in an orthogonal direction have short interacting time t with the laser light. A width (energy width) of the EIT signal, shown in FIG. 6C, formed by the light-atom interaction is inversely proportional to the interacting time t due to the uncertainty principle. Therefore, if the laser radius is small, the width (half bandwidth of detected intensity) of the EIT signal is increased, deteriorating a quality as a signal.
The width of the EIT signal formed by the light-atom interaction is inversely proportional to an electric field amplitude (intensity) of the laser light. This phenomenon is derived from that Rabi frequency is increased in proportion to an electric field. That is, if the laser light has a strong intensity, the width of the EIT signal is increased, deteriorating its quality as a signal. Further, in a case where a light receiving area of the light detector 56 is large, only a region corresponding to the laser radium is used and therefore a sufficient S/N ratio, which is essentially detected by the detector, can not be obtained.
Further, as disclosed in the first example, such structure may be acceptable that a lens is interposed between a laser light source and the gas cell on an optical path of the laser light so as to spread the laser light by the lens and evenly irradiate the metal atoms with the light, but this structure increases the number of components. Therefore, this structure is not suitable for miniaturizing of the atomic oscillator.